The traditional explanation for postnatal pulmonary edema is poor clearance of fluid from the fetal lungs in the perinatal period. Retained fluid leads to bronchioalveolar collapse with variable areas of hyperinflation and air trapping in the neonatal lung. The newborn compensates for this ventilation/perfusion mismatch with tachypnea and increased work of breathing, but often cannot completely compensate, becoming hypoxic through the period of fluid reabsorption.